Re: [PATCH v3 06/27] KVM: SVM: Fallback to flush everything if FLUSHBYASID is not available

[Yosry Ahmed]

On Wed, Oct 14, 2026 at 3:45 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> On Mon, Oct 12, 2026 at 4:36 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> >
> > Currently, if FLUSHBYASID is not available when performing a TLB flush,
> > the fallback is decrementing the ASID generation to trigger allocating a
> > new ASID. In preparation for using a static ASID per vCPU (like VMX),
> > just fallback to flushing everything if FLUSHBYASID is not available.
> > This is probably worse from a performance perspective, but FLUSHBYASID
> > has been around for ~15 years and it's not worth carrying the
> > complexity.
> >
> > Move the fallback logic into vmcb_set_flush_asid(), as more callers will
> > be added and will need the fallback as well. The only other current
> > caller is from SEV code, and no CPUs support SEV but not FLUSHBYASID (at
> > least according to AI).
>
> A more compelling argument is that sev_hardware_setup() requires FLUSHBYASID.

Huh, I didn't realize this.

>
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/svm/svm.c | 5 +----
> >  arch/x86/kvm/svm/svm.h | 5 ++++-
> >  2 files changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index ad4f172f170d9..0c3befd9bc850 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4326,10 +4326,7 @@ static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
> >          * unconditionally does a TLB flush on both nested VM-Enter and nested
> >          * VM-Exit (via kvm_mmu_reset_context()).
> >          */
> > -       if (cpu_feature_enabled(X86_FEATURE_FLUSHBYASID))
> > -               vmcb_set_flush_asid(svm->vmcb);
> > -       else
> > -               svm->current_vmcb->asid_generation--;
> > +       vmcb_set_flush_asid(svm->vmcb);
> >  }
> >
> >  static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
> > diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> > index ba5077b9838c0..3b59561d6cc2c 100644
> > --- a/arch/x86/kvm/svm/svm.h
> > +++ b/arch/x86/kvm/svm/svm.h
> > @@ -482,7 +482,10 @@ static inline void vmcb_set_gpat(struct vmcb *vmcb, u64 data)
> >
> >  static inline void vmcb_set_flush_asid(struct vmcb *vmcb)
> >  {
> > -       vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> > +       if (cpu_feature_enabled(X86_FEATURE_FLUSHBYASID))
> > +               vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> > +       else
> > +               vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;
>
> This is unnecessary. Hardware that doesn't have FLUSHBYASID only looks
> at tlb_ctl bit 0 to decide whether or not to flush the entire TLB.
> The other bits are ignored.

I think it's clearer to do the fallback since the APM doesn't
specifically call this out, or maybe it does but I can't find it.

>
> >  }
> >
> >  static inline void vmcb_clr_flush_asid(struct vmcb *vmcb)
> > --
> > 2.56.0.385.gd3acb90ef8-goog
> >